Method and apparatus for using a multi-layer multi-leaf collimation system

ABSTRACT

A multi-layer multi-leaf collimation system includes at least a two layers of collimation leaves. The first multi-leaf collimator layer is configured to primarily perform a first function to affect a radiation beam traveling from a radiation source to a target and a second multi-leaf collimator layer is configured to primarily perform a second function, different from the first function, to affect the radiation beam for the administration of a treatment plan.

TECHNICAL FIELD

These teachings relate generally to the administration of therapeuticdoses of radiation and more particularly to the use of multi-leafcollimators.

BACKGROUND

Multi-leaf collimators are comprised of a plurality of individual parts(known as “leaves”) that are formed of a high atomic numbered material(such as tungsten) that can move independently in and out of the path ofthe radiation-therapy beam in order to selectively block (and henceshape) the beam. Typically the leaves of a multi-leaf collimator areorganized in pairs that are aligned collinearly with respect to oneanother and that can selectively move towards and away from one anothervia controlled motors. A typical multi-leaf collimator has many suchpairs of leaves, often upwards of twenty, fifty, or even one hundredsuch pairs. A multi-layer multi-leaf collimator refers to a multi-leafcollimator with two or more layers of leaves positioned generallyperpendicularly along the beam path of the radiation.

By passing a therapeutic radiation beam through the aperture(s) of amulti-leaf collimator, the radiation beam can be modulated to bettermatch the dosing requirements of the treatment session. These dosingrequirements typically include (or at least presume) prescribing whichbody tissues to irradiate and which body tissues to avoid irradiating.

Common methods of delivering modulated fields for radiation treatmentinclude moving multi-leaf collimator leaves while the beam is on andother axes of the treatment delivery system are in motion. Theachievable speeds of various components of the system constrains thetreatment plan that can be administered by the system. In conventionalmulti-layer multi-leaf collimator designs, layers of leaves aretypically set close to each other to reduce obstruction and generallyhave uniform motion capabilities. As such, the motions of collimatorleaf layers do not deviate from each other substantially for treatmentplanning and treatment administration.

While a typical multi-layer multi-leaf collimator presents manybenefits, the uniform motion capabilities of conventional collimatorleaf layers are a significant constraint on the optimization andutilization of the multi-layer multi-leaf collimator and limit thesystem's treatment administration efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The above needs are at least partially met through provision of themethod and apparatus for using a multi-layer multi-leaf collimationsystem described in the following detailed description, particularlywhen studied in conjunction with the drawings, wherein:

FIG. 1 comprises a flow diagram as configured in accordance with variousembodiments of these teachings;

FIG. 2 comprises a block diagram as configured in accordance withvarious embodiments of these teachings;

FIG. 3 comprises an illustration of a collimator system as configured inaccordance with various embodiments of these teachings;

FIG. 4 comprises illustrations of intensity profiles in accordance withvarious embodiments of these teachings;

FIG. 5 comprises an illustration of a collimator system in accordancewith various embodiments of these teachings; and

FIGS. 6A and 6B comprise illustrations of projections in accordance withvarious embodiments of these teachings.

Elements in the figures are illustrated for simplicity and clarity andhave not necessarily been drawn to scale. For example, the dimensionsand/or relative positioning of some of the elements in the figures maybe exaggerated relative to other elements to help to improveunderstanding of various embodiments of the present teachings. Also,common but well-understood elements that are useful or necessary in acommercially feasible embodiment are often not depicted in order tofacilitate a less obstructed view of these various embodiments of thepresent teachings. Certain actions and/or steps may be described ordepicted in a particular order of occurrence while those skilled in theart will understand that such specificity with respect to sequence isnot actually required. The terms and expressions used herein have theordinary technical meaning as is accorded to such terms and expressionsby persons skilled in the technical field as set forth above exceptwhere different specific meanings have otherwise been set forth herein.

DETAILED DESCRIPTION

Generally speaking, pursuant to these various embodiments, an apparatusfor radiation modulation in radiation therapy includes a firstmulti-leaf collimator layer configured to primarily perform a firstfunction to affect a radiation beam traveling from a radiation source toa target. The apparatus further includes a second multi-leaf collimatorlayer configured to primarily perform a second function, different fromthe first function, to affect the radiation beam. The resultantradiation beam can then be used to administer radiation to a patientaccording to a treatment plan. (As used herein, this reference to being“configured” to primarily perform a particular function refers to morethan merely using a given layer to effect a particular function.Instead, this reference will be understood to mean that the layer itselfis specifically physically structured to effect the correspondingfunction.)

By one approach, in the apparatus for radiation modulation, the firstfunction performed by the first multi-leaf collimator layer is shapingthe radiation beam. For example, the radiation beam may be shaped byforming a first aperture according to a profile of a target area of atreatment plan to block out radiation outside of the target area.Accordingly, per this approach, the first function is to mask areas(such as organs that are near a tumor that is the target of thetreatment) that are to be protected from the radiation, By one approach,in the apparatus for radiation modulation, the second function performedby the second multi-leaf collimator layer is modulating a fluencedistribution of the radiation beam (for example, within the intendedtreatment volume). For example, fluence distribution may be modulated bymodulating a second aperture to vary radiation intensities in differentregions within a target area according to a treatment plan.

By one approach, in the apparatus for radiation modulation, the firstmulti-leaf collimator layer substantially differs from the secondmulti-leaf collimator layer in one or more of leaf transmission,penumbra width, maximum leaf speed, and median leaf width. For example,the first multi-leaf collimator layer may have leaves with widthsbetween 2 mm and 2 cm and the second multi-leaf collimator layer mayhave leaves with widths between 5 mm and 5 cm.

In lieu of the foregoing or in combination therewith, an apparatus forradiation modulation in intensity modulated radiation treatment includesa first multi-leaf collimator layer having a first set of leavesconfigured to affect a radiation beam traveling from a radiation sourceto a target and a second multi-leaf collimator layer having a second setof leaves configured to affect the radiation beam along with the firstmulti-leaf collimator layer. By one approach the median width of thesecond set of leaves is substantially larger than the median width ofthe first set of leaves. The first multi-leaf collimator layer and thesecond multi-leaf collimator layer may be configured to primarilyperform different functions.

In lieu of the foregoing or in combination therewith, a method forradiation modulation in radiation therapy includes the steps ofcontrolling a first multi-leaf collimator layer being an integral partof a discrete multi-leaf collimator to primarily perform a firstfunction to affect a radiation beam traveling from a radiation source toa target, and controlling a second multi-leaf collimator layer alsobeing an integral part of the discrete multi-leaf collimator toprimarily perform a second function, different from the first function,to affect the radiation beam.

So configured, in one or more of these approaches, the movements ofdifferent layers of a multi-layer multi-leaf-collimator are optimizedfor different functions to collectively administer a treatment plan. Theleaves in each layer may be optimized according to that layer's specificmaximum speed constraint for layer-specific functions. For example, alayer with a lower maximum speed may be used primarily for conformalapertures and a faster layer may be used to produce fast modulation. Bysome approaches, a multi-layer multi-leaf collimator may be used foradaptive treatment delivery by using the first layer to modulate thebeam intensity profile to a region somewhat larger than the expectedtarget projection and using the second layer to restrict the aperture tothe target projection at the time of the treatment. These approachesgenerally improve upon conventional processes by increasing theoptimization and operational efficiency of a multi-layer multi-leafcollimator.

These and other benefits may become clearer upon making a thoroughreview and study of the following detailed description. Referring now tothe drawings, and in particular to FIG. 1 , an illustrative process 100that is compatible with many of these teachings is presented below. Forthe sake of an illustrative example, it is presumed herein that acontrol circuit of choice carries out the steps, actions, and/orfunctionality of this process 100. FIG. 2 presents an illustrativeexample in this regard.

As shown in FIG. 2 , a radiation therapy treatment platform 200 caninclude or otherwise operably couple to a control circuit 201. Being a“circuit,” the control circuit 201 therefore comprises structure thatincludes at least one (and typically many) electrically-conductive paths(such as paths comprised of a conductive metal such as copper or silver)that convey electricity in an ordered manner, which path(s) will alsotypically include corresponding electrical components (both passive(such as resistors and capacitors) and active (such as any of a varietyof semiconductor-based devices) as appropriate) to permit the circuit toeffect the control aspect of these teachings.

Such a control circuit 201 can comprise a fixed-purpose hard-wiredhardware platform (including but not limited to an application-specificintegrated circuit (ASIC) (which is an integrated circuit that iscustomized by design for a particular use, rather than intended forgeneral-purpose use), a field-programmable gate array (FPGA), and thelike) or can comprise a partially or wholly-programmable hardwareplatform (including but not limited to microcontrollers,microprocessors, and the like). These architectural options for suchstructures are well known and understood in the art and require nofurther description here. This control circuit 201 is configured (forexample, by using corresponding programming as will be well understoodby those skilled in the art) to carry out one or more of the steps,actions, and/or functions described herein. It will also be understoodthat a “control circuit” can comprise multiple such components orplatforms as well as suggested by the phantom control circuit box inFIG. 2 .

By one optional approach the control circuit 201 operably couples to amemory 202. This memory 202 may be integral to the control circuit 201or can be physically discrete (in whole or in part) from the controlcircuit 201 as desired. This memory 202 can also be local with respectto the control circuit 201 (where, for example, both share a commoncircuit board, chassis, power supply, and/or housing) or can bepartially or wholly remote with respect to the control circuit 201(where, for example, the memory 202 is physically located in anotherfacility, metropolitan area, or even country as compared to the controlcircuit 201).

In addition to radiation treatment plans, this memory 202 can serve, forexample, to non-transitorily store the computer instructions that, whenexecuted by the control circuit 201, cause the control circuit 201 tobehave as described herein. (As used herein, this reference to“non-transitorily” will be understood to refer to a non-ephemeral statefor the stored contents (and hence excludes when the stored contentsmerely constitute signals or waves) rather than volatility of thestorage media itself and hence includes both non-volatile memory (suchas read-only memory (ROM) as well as volatile memory (such as anerasable programmable read-only memory (EPROM).)

The radiation therapy treatment platform 200 also includes a radiationsource 203 that operably couples and responds to the control circuit201. So configured, the corresponding radiation beam 204 as emitted bythe radiation source 203 can be selectively switched on and off by thecontrol circuit 201. These teachings will also accommodate having thecontrol circuit 201 control the relative strength of the radiation beam204. Radiation sources are well understood in the art and require nofurther description here.

The radiation beam 204 is directed towards a multi-layer multi-leafcollimation system 205 that also operably couples to the control circuit201 to thereby permit the control circuit 201 to control the movement ofthe collimation systems leaves and hence the formation and distributionof one or more beam-shaping and radiation-modulating apertures. Theresultant modulated radiation beam 206 then reaches a treatment targetin a corresponding patient 207.

FIG. 3 presents a representative view a multi-layer multi-leafcollimator 300 of the multi-layer multi-leaf collimation system 205according to some embodiments. The proximal layer 302 and distal layer303 of the multi-layer multi-leaf collimator 300 are generallyjuxtaposed one atop the other with some amount of intervening spacebetween the layers. In some embodiments, the intervening space betweenlayers may range from 0.5 cm to a few centimeters. In some embodiments,the intervening space between layers is approximately 1 cm. The proximallayer 302 is oriented towards the radiation source 301 (and hence isrelatively “proximal” to the radiation source 301) and the distal layer303 is oriented opposite the radiation source 301 (with respect to theproximal layer 302) and towards the target 304 such as a patient.Generally speaking, this proximal layer 302 includes a plurality ofselectively movable collimating leaf pairs each including a first leafand a second leaf. So configured, when one or both collimating leaves ofa pair of collimating leaves are selectively moved away from oneanother, a beam-shaping aperture forms therebetween. (The manner bywhich electric motors can be employed to effect such movement comprisesa well-understood area of prior art endeavor. Accordingly, for the sakeof brevity, additional details in those regards are not provided here.)

The distal layer 303 generally also includes pairs of collimatingleaves, similar to the proximal layer 302. However, the proximal layer302 and the distal layer 303 substantially differ in one or more of leaftransmission, penumbra width, maximum leaf speed, and median leaf width.For example, the distal layer 303 may comprise leaves with widthsbetween 2 mm and 2 cm and the proximal layer 302 may comprise leaveswith widths between 5 mm and 5 cm. In some embodiments, the median leafwidth of the distal layer 303 may be 1.5-2.5 times larger than themedian leaf width of the proximal layer 302. In some embodiments, themedian leaf width of the proximal layer 302 may be 1.5-2.5 times largerthan the median leaf width of the distal layer 303. Due to thedifference in the dimensions between the multi-leaf collimator layers,the layers also differ in dosimetric properties and motion capabilities(e.g. maximum speed).

So configured, the distal layer 303 may be used, at least mainly, toproduce conformal apertures that conform to the shape of the targetvolume with the primary purpose of blocking radiation entirely incertain areas. (As used herein to refer to the functionality and use ofthe multi-leaf collimator layers, terms such as “substantially” and“mainly” shall be understood to mean more than fifty percent of thecorresponding functionality/usage of the respective layer. In someapplication settings it may be appropriate to require or specify largerpercentages, such as at least sixty percent, seventy percent, eightypercent, ninety percent, or even one hundred percent.)

The proximal layer 302 may be used conversely, at least mainly, forin-field modulation (i.e., within the field of the target volume). Theaperture of the proximal layer 302 may be optimized based on a targetprojection of the treatment area and the distal layer 303 may beoptimized based on a fluence map describing the relative portions ofradiation passing through different areas of the target projection. Thefluence map may be later converted into a leaf sequence of the distallayer 303. In some embodiments, one of the layers may be controlled tocreate dose rate modulation together with the varying distal leafpositions.

FIG. 4 presents an illustration of a radiation modulating process usinga multi-layer multi-leaf collimator 300. Radiation intensity profiles ofa radiation beam before and after the beam passes the local dose ratecontrol layer 401 and the conformal shaping layer 402 are shown. In someembodiments, the local dose rate control layer 401 may comprise theproximal layer 302 and the conformal shaping layer 402 may comprise thedistal layer 303, or vice versa.

The intensity profiles 410, 411, and 412 represent radiation intensitiesin the horizontal direction along one leaf. The arrows below the profileimages correspond to the target range for treatment. The first profile410 illustrates an unmodified beam from a radiation source. The secondprofile 411 illustrates the intensity profile of the radiation beamafter the beam passesthrough the local dose rate control layer 401. Atthis stage, the profile height is modified, but the width of theradiation beam is not modified in the horizontal direction (direction ofthe leaves) and does not match the target range. The third profile 423shows the radiation intensity profile after the beam has passed throughboth the local dose rate control layer 401 and the conformal shapinglayer 402. The profile height is not further modified by the conformalshaping layer 402. The width of the resulting radiation intensityprofile 412 now matches the target range.

In the 2D radiation intensity profiles 421, 422, and 423, lightershadings corresponding to higher radiation intensity and darker shadingscorrespond to lower radiation intensity. The target projection 430represents a treatment area according to a treatment plan. The leaves ofthe local dose rate control layer 401 and the conformal shaping layer402 travel in the horizontal direction relative to the 2D intensityprofiles shown in FIG. 4 .

Intensity profile 421 represents the distribution of radiation intensityimmediately after the source where radiation intensity is generallyuniform in the field. Intensity profile 422 represents the distributionof radiation intensity after the beam passes through the local dose ratecontrol layer 401. In profile 422, radiation intensity distribution ismodified in stripes along the leaf direction. The number of stripes inthe profile corresponds to the number of leaves in the local dose ratecontrol layer 401. Intensity profile 423 represents the distribution ofradiation intensity after the beam passes through the conformal shapinglayer 402. In profile 423, the radiation intensity outside the targetprojection 430 is blocked by the shaping layer 402 and the intensitymodulated stripes are visible within the target projection 430 region.

In some embodiments, the above optimization principle can be applied todifferent multi-leaf collimator designs. For example, the roles ofdistal and proximal layers can be switched and the division of roles canbe different from above. In some embodiments, a multi-leaf collimatormay have more than two layers and the treatment planning system mayoptimize each layer to primarily perform a different role such as doserate control, beam modulation, and conformal shaping.

In some embodiments, the above approach is well suited for treatmentswhere the beam direction relative to the patient is moving while thebeam is on, such as in a conventional arc field system where the beamdirection is changed by a rotating gantry. Typically, thedegrees-of-freedom of aperture shaping varies more slowly than thedegrees-of-freedom of field modulation. By these approaches,optimization efficiency is improved due to the amount of optimizeddegrees-of-freedoms (corresponding spatial variation of leaf positionsand dose rate parameters) being split between two different optimizationtasks.

FIG. 5 presents another representative view of a multi-layer multi-leafcollimator 500 for the multi-layer multi-leaf collimation system 205according to some embodiments. The multi-layer multi-leaf collimator 500includes a shaping layer 502 and a modulation layer 503 positionedbetween a source 501 and a patient 504. The shaping layer 502 isproximal to the source 501 while the modulation layer 503 is distal tothe source 501.

Conventional radiation therapy treatment is based on delivering the sametreatment plan multiple times. However, changes can occur between thecapture of planning images and the administration of the treatment plan.The general approach is to take this uncertainty in treatment-timepatient anatomy into account by adding margins to the delineatedstructures at planning time. Images taken just prior to the treatmentcan also be used for on-line adaptation of the plan.

The multi-layer multi-leaf collimator 500 may be used for adaptivetreatment delivery by using the modulation layer 503 to modulate thebeam intensity profile to a region somewhat larger than the expectedtarget projection and using the conformal shaping layer 502 to restrictthe aperture to the target projection at the time of the treatment. Bythis approach, the adaptation of the treatment plan may only include thereshaping of the aperture in the second layer. A 3D image reconstructionmay be eliminated since target projection may be determined from thedirections of the fields alone. As such, plans generated using theproposed approach can be adapted quickly, using only 2D images ifnecessary. These approaches also reduce the requirements for qualityassurance since the dynamic part of the sequence (e.g. the motion of theshaping layer 502) is not changed at treatment time. The technique isalso an improvement from simple isocenter shift since it can providemore uniform dose distribution and the plan can be adapted to varioustarget shape changes and not only translations. While the modulationlayer 503 is shown as the distal layer in FIG. 5 , in some embodiments,the dynamic modulation layer 503 could either be the layer closer to thesource of radiation (hence the proximal layer) or the layer closer tothe patient (hence the distal layer).

FIGS. 6A and 6B represent beam-eye views of an intensity-modulatedradiation therapy (IMRT) field. FIG. 6A represents a treatment plancreated based on a target projection 601, which may be based on planningimages captured prior to the treatment. A target projection region withadaptation margin 602 is determined based on the planning image of thetarget projection 601. A region of dynamic modulation 603 is thendetermined to encompass the target projection with adaptation margin602. FIG. 6B represents the adaptations at the time of the treatment. InFIG. 6B, the actual target projection 601′ deviates from the planningimage of the target projection 601. To adapt to this change, the shapingaperture 611 is adjusted based on the actual target projection 601′. Insome embodiments, the shaping layer 502 of the collimator is controlledto define the region of dynamic modulation 603 during treatment, whilethe modulation layer 503 is controlled to create the adapted shapingaperture 611 in response to changes of the target projection 601′. Thesystem may further monitor the adapted shaping aperture 611 to ensurethat it remains within the region of dynamic modulation 603 duringtreatment.

By this approach, the leaf sequence for the IMRT field uses the dynamiclayer leaf motion to yield a uniform dose distribution for a targetvolume with an extended 3D margin as shown in FIGS. 6A and 6B. The sizeof the margin region may be determined based on the expected movement ofthe target. During the planning process, the shaping layer aperture canbe set to follow the extended region of dynamic modulation 603 to definethe area where the adaptation may be done. Quality assurance may beperformed to verify that the machine is able to reproduce the dynamicleaf pattern with sufficient dosimetric accuracy.

At the time of treatment, when the actual target projection isdetermined, the aperture of the shaping layer is reduced to therecognized treatment time aperture. The shaping aperture adaptation maybe carried out while the gantry is moving into position. In someinstances, adaptation may be needed on only one side of the target, suchas the side proximal to a critical organ. Additionally, adaptation maybe performed based on 2D images (such as kV images) from the directionof the field without reconstructing a 3D CT image.

In some embodiments, a treatment plan may be created using the actualtarget with the aforementioned extended 3D margin if the delivery methoddescribed above is created using inverse planning optimization. Whenoptimization is done this way, the dose in critical organs is biased tobe larger than the value actually delivered since the adaptive actionreduces the shaping aperture and decreases the total dose delivered. Thebias may be reduced by taking into account that the expected doseaccumulation in the target extension volume decreases with distance fromthe surface of the non-extended target. For example, critical organdosage can be determined based on an expected dose distributioncalculated by reducing the fluence value in the extended apertureregion. By some approaches, the adaptive treatment can provide morerobust results if dose robustness is further taken into account in theoptimization time by adding a bias to the optimization to prefer smoothleaf sequences in the dynamic layer. The plan can further be optimizedto use beam directions that are also good imaging directions.

In some embodiments, various methods and apparatuses described hereinmay also be applied to Volumetric Arc Therapy (VMAT) fields. In VMATfields, both collimator layers would be dynamic but the shaping layerleaves would have a slower motion and be configured to follow the targetprojection.

With continued reference to the foregoing illustrations, and inparticular to FIGS. 1 and 2 , process 100 provides radiation modulationin radiation therapy. As is well understood in the art, generating aradiation treatment plan typically relies upon models for one or moreaspects of the radiation therapy treatment platform. It is also wellunderstood that generating a radiation treatment plan often entailsmaking use of iterative optimization processes. As used herein,“optimization” will be understood to refer to improving a treatment planwithout necessarily ensuring that the optimized result is, in fact, thesingular best solution. Such optimization often includes automaticallyadjusting one or more treatment parameters (often while observing one ormore corresponding limits in these regards) and mathematicallycalculating a likely corresponding treatment result to identify a givenset of treatment parameters that represent a good compromise between thedesired therapeutic result and avoidance of undesired collateraleffects. Since optimization practices themselves are a well-understoodarea of prior art endeavor, further details are not provided hereinthese regards for the sake of brevity and simplicity.

By some approaches, the steps of FIG. 1 are performed with a multi-layermulti-leaf collimation system of a radiation therapy treatment platform.In some embodiments, the multi-layer multi-leaf collimation system maycomprise one or more of the multi-layer multi-leaf collimation system205, the multi-layer multi-leaf collimator 300, and/or the multi-layermulti-leaf collimator 500. By some approaches, the multi-layermulti-leaf collimation system comprises a first multi-leaf collimatorlayer and a second multi-leaf collimator layer. The first multi-leafcollimator layer may substantially differ from the second multi-leafcollimator layer in one or more of leaf transmission, penumbra width,maximum leaf speed, and median leaf width. In some embodiments, themedian width of the second set of leaves is substantially larger than amedian width of the first set of leaves. In some embodiments, the firstmulti-leaf collimator layer is configured to primarily perform a firstfunction to affect a radiation beam traveling from a radiation source toa target and the second multi-leaf collimator layer is configured toprimarily perform a second function, different from the first function,to affect the radiation beam.

In step 101, the control circuit retrieves a treatment plan for themulti-layer multi-leaf collimation system. In some embodiments, thetreatment plan uses each layer to primarily perform a different rolesuch as dose rate control, beam modulation, and conformal shaping. Insome embodiments, the treatment plan uses one layer of the multi-leafcollimator to define a treatment area based on a planning image and anadaptation margin and uses a second layer for adaptation. In someembodiments, motions of each layer are optimized for the treatment planbased on their individual maximum speed capability.

In step 102, the control circuit controls the first multi-leafcollimator layer that comprises a part of a discrete multi-leafcollimator to primarily perform a first function to affect a radiationbeam traveling from a radiation source to a target. In some embodiments,the first function comprises shaping the radiation beam by forming afirst aperture according to a profile of a target area of a treatmentplan to block out radiation outside of the target area. The aperture maycorrespond to a planning image with an adaptation margin.

In step 103, the control circuit controls a second multi-leaf collimatorlayer that also comprises an integral part of the discrete multi-leafcollimator to primarily perform a second function, different from thefirst function, to affect the radiation beam. In some embodiments, thesecond function comprises modulating a fluence distribution of theradiation beam by modulating a second aperture to vary radiationintensities in different regions within a target area according to atreatment plan. In some embodiments, the second function comprisesadaptively shaping the aperture to the target projection at the time ofthe treatment.

By some approaches, steps 102 and 103 may be carried out simultaneouslyor in close succession to coordinate the movements of the multi-leafcollimator layers and other components of the system. Steps 102 and 103may be performed repeatedly during a treatment session from differentangles to carry out a treatment plan.

It shall be understood that the foregoing process 100 is quite flexiblein practice and can be modified to accommodate variations,modifications, or even substitutions as regards the foregoing details.By some approaches, the first multi-leaf collimator layer may comprise adistal layer 303 and the second multi-leaf collimator layer may comprisea proximal layer 302, or vice versa. By some approaches, the firstmulti-leaf collimator layer may comprise the shaping layer 502 and thesecond multi-leaf collimator layer may comprise the modulation layer503, or vice versa. By some approaches, the first multi-leaf collimatorlayer may have a larger leaf transmission, penumbra width, maximum leafspeed, and/or median leaf width as compared to the second multi-leafcollimator layer, or vice versa.

Those skilled in the art will recognize that a wide variety ofmodifications, alterations, and combinations can be made with respect tothe above-described embodiments without departing from the scope of theinvention, and that such modifications, alterations, and combinationsare to be viewed as being within the ambit of the inventive concept.

What is claimed is:
 1. A method for radiation modulation in radiationtherapy comprising: providing a first multi-leaf collimator layer thatis disposed proximal to a radiation source and that comprises a part ofa discrete multi-leaf collimator, the first multi-leaf collimator layerbeing configured to primarily perform a first function as regardsaffecting a radiation beam traveling from the radiation source to atarget and wherein leaves of the first multi-leaf collimator layer areconstrained to a first maximum speed; providing a second multi-leafcollimator layer that is positioned distal to the radiation source andthat also comprises an integral part of the discrete multi-leafcollimator, the second multi-leaf collimator level being configured toprimarily perform a second function as regards affecting the radiationbeam that is different from the first function, and wherein leaves ofthe second multi-leaf collimator layer are constrained to a secondmaximum speed that is higher than the first maximum speed; andoptimizing a radiation treatment plan that utilizes both of the firstand second multi-leaf collimator layers wherein the leaves in each ofthe first and second multi-leaf collimator layers are optimizedaccording to each layer's specific maximum speed constraint forlayer-specific functions, such that motions of each layer are optimizedfor the radiation treatment plan based on their individual maximum speedcapability.
 2. The method of claim 1 wherein the first functioncomprises shaping the radiation beam to conform to a target area.
 3. Themethod of claim 1 wherein controlling the first multi-leaf collimatorlayer comprises forming a first aperture according to a profile of atarget area of a treatment plan to block out radiation outside of thetarget area.
 4. The method of claim 1 wherein the second functioncomprises modulating a fluence distribution of the radiation beam. 5.The method of claim 1 wherein controlling the second multi-leafcollimator layer comprises modulating a second aperture to varyradiation intensities in different regions within a target areaaccording to a treatment plan.
 6. The method of claim 1 wherein thefirst multi-leaf collimator layer further substantially differs from thesecond multi-leaf collimator layer in one or more of leaf transmission,penumbra width, and median leaf width.